Implementing multicasting

ABSTRACT

An arrangement and method for implementing multicasting in IP networks, in which multicast packets are transmitted along a multicast tree from one transmitter through several multicast controllers to several recipients is discussed. In the method at least one multicast tree intended for control messages is generated in the network from a network multicast controller to cell-level multicast controllers. The network multicast controller transmits along the multicast tree control messages to the cell-level multicast controllers. The control messages contain information on the network multicast and a command to connect to the network multicast tree intended for multicasts.

This application is a Continuation of International ApplicationPCT/FI02/00719 filed Sep. 6, 2002 which designated the U.S. and waspublished under PCT Article 21(2) in English.

FIELD OF THE INVENTION

The invention relates to an arrangement and method for implementingmulticasting in IP networks in particular.

BACKGROUND OF THE INVENTION

Multicasting is a method, by which one and the same content can bedistributed efficiently to several recipients in a network. Incomparison with conventional transmission between two parties, referredto by the abbreviation PTP (point to point), multicasting savesbandwidth, because in IP networks, for instance, packets with anidentical content are not transmitted from one source to severalrecipients over the entire route, but one packet is transmitted from thesource and multiplied in the last possible router in the network into asmany copies as there are recipients. Thus, this is a PTM (point tomultipoint) transmission. This transmission can be simultaneous,contrary to PTP transmission, in which the transmission is to onerecipient at a time.

A certain section of the IP address space is reserved for multicastaddresses and these addresses are not given as normal IP addresses ofdevices. A given multicast address belongs to all devices belonging tothe group in question. The basic idea in multicasting is that therecipients register to desired groups by using a known protocol(Internet Group Management Protocol, IGMP). Separate multicastingprotocols deliver a multicast transmission (UDP-based traffic from oneto many) from the sender to the subscribers.

Multicasting is implemented by means of multicast trees. Each multicastsender has a multicast tree. A multicast tree refers to a connectionfrom a sender through different routers and branching off from therouters to each recipient.

The known method for implementing multicasting causes problems in newdata networks. This concerns especially networks that compriseunidirectional links. One such network is the multi-bearer network (MBN)that is a network arrangement providing several different networkservice types for the delivery of data packets. The network comprises acore network that is connected to the Internet or some other public datanetwork and through interface units to different network services, i.e.access networks, such as the cellular radio networks GSM (Global Systemfor Mobile Communication), GPRS (General Packet Radio System) and UMTS(Universal Mobile Telecommunications System), and the broadcast networksDAB (Digital Audio Broadcast) and DVB (Digital Video Broadcast). Of theabove networks, the cellular radio networks are bi-directional and thebroadcast networks unidirectional.

A subscriber terminal connected to a system through a unidirectionallink cannot connect to a multicast transmission organized by using theprior art, because it is unable to transmit anything to the network dueto the unidirectional link that has no transmit capability.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to implement a method and anarrangement implementing the method in such a manner that multicastingis possible in different types of networks that also compriseunidirectional links. This is achieved by a method for implementingmulticasting in IP networks, in which multicast packets are transmittedby means of a multicast tree from one transmitter through severalmulticast controllers to several recipients. In the method of theinvention, at least one multicast tree intended for control messages isgenerated in the network from a network multicast controller tomulticast controllers at cell level, the network multicast controllertransmits control messages along the multicast tree to the cell-levelmulticast controllers, and the control messages contain information onthe multicast transmission of the network and a command to connect tothe multicast tree of the network intended for multicasts.

The invention also relates to an arrangement for implementingmulticasting in IP networks that comprises a number of routerstransmitting messages of the different components in the network to eachother, at least one multicast transmitter that is arranged to transmitmulticast packets through a multicast tree to several receivers, anumber of cell-level multicast controllers that are arranged to transmitpackets to receivers, a multicast controller that is arranged to controlthe cell-level multicast controllers. In the system of the invention,the network comprises at least one multicast tree intended for controlmessages from the network multicast controller to the cell-levelmulticast controllers, the network multicast controller is arranged totransmit control messages along the multicast tree to the cell-levelmulticast controllers, and the control messages contain information onthe multicast transmission of the network and a command to connect tothe multicast tree of the network intended for multicast transmissions.

Preferred embodiments of the invention are described in the dependentclaims.

One basic idea of the invention is to use multicast trees both fortransmitting control data from the multicast controller and fortransmitting the actual information from the signal source. Thus in apreferred embodiment, the network has two logically different multicasttrees, one for control messages (control announcement tree, CAT) and onefor other multicast messages (Internet standard multicast, ISM). Eachtree is identified by the IP multicast address of the tree.

The CAT multicast tree transmits control messages from the networkmulticast controller to the cell-level multicast controllers. Thecontrol messages comprise information on the multicast transmission ofthe network and commands to connect to the multicast tree of the networkintended for multicast transmissions. The cell-level multicastcontrollers of the network connect to the multicast tree intended forthe control messages of the network when they connect to the IP network.

The method and system of the invention provide several advantages. Inthe solution of the invention, each recipient need not separatelyregister as a recipient, but the cell-level multicast controllerregisters to receive and transmit multicast transmissions in its cellregardless of whether any one of the terminals in the cell hasregistered as a recipient of the multicast transmissions. This way, aterminal with a unidirectional link, for instance, can receive multicasttransmissions even though it cannot transmit a request to the networkand thus not register as a recipient of multicast transmissions.

Further, the network multicast controller need not know and exactlyidentify the cell-level multicast controllers. Therefore, an increase inthe number of cell-level multicast controllers does not affect in anyway the operation of the multicast controller of the actual network, nordoes it cause a scaling problem in the amount of maintenance andconfiguration data.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in greater detail by means ofpreferred embodiments and with reference to the attached drawings, inwhich

FIGS. 1A to 1C illustrate multicast trees,

FIG. 2 shows an example of an IP network, to which the solution of theinvention can be applied,

FIG. 3 shows a second example of an IP network,

FIG. 4 shows by means of a signal diagram an example of the signallingof IP network components with respect to multicasting, and

FIG. 5 illustrates different access networks.

DETAILED DESCRIPTION OF THE INVENTION

Let us examine the examples of FIGS. 1A to 1C of IP networks andmulticast trees. The figures show two multicast transmitters 100, 102,two recipients 104, 106 and a number of routers 108 to 118, throughwhich packets transmitted by the transmitters pass in the IP network.The routers 108 to 118 are, in a way, programmed switches that have anumber of input ports and a number of output ports and that followingcertain predefined rules and using the address fields in the packetstransmit the packets from the input ports to given output ports. Inprinciple, there are two types of multicast trees. Shortest path trees,i.e. source trees, are illustrated in FIGS. 1A and 1B. FIG. 1A shows theshortest path through routers from a transmitter 100 to recipients 104and 106, and FIG. 1B shows the shortest path through routers from atransmitter 102 to recipients. The shortest path tree thus uses a routethat runs through as few routers as possible.

FIG. 1C in turn illustrates a shared distribution tree. Here, thetransmitters 100 and 102 transmit packets to a certain router 112 byusing the shortest path method, and from there onwards, the packets goalong a common route. The router 112 is called a rendezvous point RP.From it onwards, the route is divided between the packets of severalmulticast transmitters. The shortest path method requires more memory,but minimizes the delay in packet propagation. The shared distributiontree in turn uses less memory, but the delay of the packets may belonger, because the route of the packets is not necessarily the shortestpossible. The above alternatives are equal for the present invention.

A few multicast methods have been developed for IP networks that can beapplied to the preferred embodiments of the invention. Such methodsinclude the protocol independent multicast-sparse mode PIM-SM andprotocol independent multicast-dense mode PIM-DM. PIM-SM is designed fornetworks, in which groups are relatively sparsely situated, and PIM-DMfor networks, in which groups are in an area where receivers arerelatively densely situated.

Let us then examine FIG. 2 that shows an example of an IP network, towhich the solution of the invention can be applied. The networkcomprises three layers. A backbone layer 200 comprises the backbone ofthe network or a number of backbones of different networks that areinterconnected through routers and that are capable of transmittingpackets and generating a multicast tree, i.e. that comprise PIM-SMrouters 206 to 210, for instance. An interface unit layer (IU layer) 202comprises interface functions between the core network and the accessnetwork. This layer can also be implemented as a logical layer in such amanner that the interface functions are implemented either in theequipment of the core network or the access network. An access networklayer 204 comprises an access network, by means of which users areconnected through the core network to IP-based services.

Let us now examine some of the network elements shown in the figure. Amulticast source 212 transmits multicast data in packets to the IPnetwork. PIM routers 206 to 210 transmit the multicast packets along themulticast trees from one router to another. Designated routers 214, 216are connected to one or more cell-level controllers. A designated routeris the closest router capable of multicasting to a cell-levelcontroller. A cell-level controller 218 to 222 controls one or morecells in the access network. Depending on the system, a cell-levelcontroller can control all cells in the system, or there may be onecontroller per cell.

A network multicast controller 224 controls the multicasting of thesystem. The network multicast controller 224 can thus be a differentdevice than the multicast transmitter 212. Cell-level multicastcontrollers 226 to 230 are together with cell-level controllers. Bothreside in the access network layer. The cell-level multicast controllerskeep a record on the active transmissions in a cell. The cell-levelmulticast controllers connect to the multicast tree intended for thenetwork control messages as described later.

Let us next examine FIG. 3 that shows an example of an IP network, towhich the solution of the invention can be applied, and multicast treesolutions of a preferred embodiment. The figure shows by way of examplethree cell-level controllers 300 to 304 that herein also comprisecell-level multicast controllers. The cell-level controllers areconnected through designated routers 306 to 310 to the rest of thenetwork. In a solution of a preferred embodiment, the network has twologically different multicast trees, one for control messages (controlannouncement tree, CAT) and one for other multicast messages (Internetstandard multicast, ISM). The control message tree ends at the networkmulticast controller 224, and it is marked with a dashed line in thefigure. The multicast tree of the multicast messages ends at themulticast transmitter 212, and it is marked with a continuous line inthe figure. As shown in the figure, multicast trees can be different,which is due to the fact that the multicast transmitter 212 and thenetwork multicast controller 224 may reside in different parts of thenetwork.

As mentioned earlier, a certain section of the IP address space isreserved for multicast addresses and these addresses are not given asnormal IP addresses of devices. A given multicast address belongs to alldevices belonging to the group in question. D-group addresses in therange from 224.0.0.0 to 239.255.255.255 are allocated for multicasting.The address thus identifies the group and not an individual recipient.

Let us next examine a solution of a preferred embodiment by means of thesignalling diagram of FIG. 4. The diagram shows IP network componentsand the signalling between them from the viewpoint of multicasting. Itis assumed herein that in the first step, the cell-level controller,which comprises a cell-level multicast controller, is connected to thenetwork. The cell-level multicast controller then transmits a message400 to the closest designated router for the purpose of connecting tothe multicast tree intended for network control messages. This occursregardless of whether the cell-level controller has received anymulticast reception requests from the terminals in its cell. The routertransmits the message 402, 404 on through PIM routers to the multicastcontroller. The message route in question becomes a multicast tree fromthe multicast controller to the cell-level controller. The message ofthe cell-level controller is transmitted to the multicast controller asIGMP connection messages 400 to 404. IGMP (Internet group managementprotocol) is a known multicasting protocol.

In the second step, the multicast controller transmits control messagesto the control announcement tree (CAT) for cell-level multicastcontrollers. The messages can be transmitted as SAP notifications, forinstance, comprising SDP multicast descriptions. SAP (sessionannouncement protocol) is a known protocol for multicast messagetransmission and it is described in reference publication Handley M,“Session Announcement Protocol,” IETF RFC 2974, October 2000. SDP inturn is a message description that is described in reference publicationHandley M and Jacobson V, “SDP: Session Description Protocol,” IETF RFC2327, April 1998. Each CAT is identified by one multicast address, soSAP packets in one multicast tree have the same multicast address.Messages propagate in the multicast tree 406 to 408 until the cell-levelcontroller.

A control message can comprise the following fields:

-   -   One or more multicast group identifiers.    -   A recipient definition filter. Multicasting can for instance be        intended for all receivers in a certain area A, or for all        receivers with a certain property.    -   The time during which the information contained in the control        message is valid.    -   Sender authentication.    -   A request for acknowledgement, if the multicasting defined by        the message will not be received.

The multicast group identifier field is obligatory. The other fields areoptional. Sender authentication can be arranged with the followinggenerally known method, for instance: the network multicast controllercalculates from the control message a standard-length result ‘messagedigest’ with a secure hash function. This result is signed digitallywith a private key of the network multicast controller. The finalresult, a message authentication code, is attached to the controlmessage. When a cell-level multicast controller receives a controlmessage, it first separates the authentication code. The code is openedwith a public key of the network multicast controller (each cell-levelmulticast controller has this key or it can easily be obtained, since apublic key is public information). Next, the ‘message digest’ iscalculated from the message with the hash function and it is comparedwith the opened code. If the results match, the control message isauthentic (it comes from an authentic source) and whole (it has notchanged underway).

In the third step, the cell-level controllers receive the controlmessages. Depending on the content of the message and the configurationof the cell-level multicast controller, the cell-level controlleroperates in different ways when receiving a message. The cell-levelcontroller can do the following, for instance:

-   -   Register as a multicast recipient to a multicast tree by using        an IGMP join command, for instance, 412 to 416.    -   As above, but in addition it can notify 418 the terminals in the        cell that a multicast transmission is available (AVAILABLE) by        using an SDP message, for instance.    -   As in the first alternative, but it can notify 418 the terminals        in the cell that the multicast transmission must be received        (MUST-LISTEN). The message can be an SDP message, for instance.    -   It can leave the message unprocessed. This can occur, for        instance, if the time indicated in the message has elapsed or if        there is no certainty about the sender. Further, if the message        indicates that the recipient group has been limited with a        filter, and the recipient does not belong to this group, the        message is not processed.

In the fourth step, the multicast transmitter transmits multicastpackets from the core network to the cells along the multicast tree 420to 426.

The invention has several different embodiments. For instance, theprotocol used for multicasting in the network, with which the multicasttrees and messages are transmitted, is not significant. Theabove-mentioned PIM-SM and PIM-DM protocols, for instance, can be used.There are also several alternatives for the used control protocol, suchas SDP over SAP and UDP (User Datagram Protocol) or ICMP (InternetControl Message Protocol). SAP, UDP and ICMP are generally usedprotocols in IP networks.

FIG. 5 illustrates an IP-based multi-bearer network, to which differentaccess networks are connected. The routers 306 to 310 shown in thefigure correspond to the routers in FIG. 3. Router 306 is connected to agateway GPRS Support node GGSN 500 of the GPRS (General Packet RadioSystem) network. GGSN 500 routes packets between the GPRS system and theIP network external to it. GGSN further has a connection 502 to aserving GPRS support node SGSN 504. The main task of SGSN 504 is totransmit and receive packets to and from user equipment 506 supportingpacket-switched transmission by using a base station system. The basestation system comprises a base station controller BSC 508 and a basestation BS 510 controlled by it. In this case, the cell-level controlleris in SGSN 504.

The router 308 is connected to a WLAN (Wireless Local Area Network)router 512. The router 512 in turn has a connection 513 to a router 514.The router 514 in turn has a connection to an access point AP 516 thathas a wireless connection to a terminal 518. In this case, thecell-level controller is either in the router 512 or in the router 514.

The router 310 is connected to a DAB/DVB system router 520. The router520 is connected to a coder 522, in which the coding required by thesystem is performed, and the coder is connected to a multiplexer 524,from which a signal is transmitted through an ATM network to thetransmitter 528 that transmits the signal on a unidirectional connectionto a terminal 530. In this case, the cell-level controller is in therouter 520.

The above DAB/DVB description is only one example of a possibleimplementation. Said network can also be implemented in other ways, suchas by having the coder and/or multiplexer in the transmitter 528,whereby the network comprises consecutive routers ending up in thetransmitter 528. The cell-level controller then always resides in thepenultimate router.

In all the examples described above, the cell-level controller is thusarranged to connect to the multicast tree intended for network controlmessages when it connects to the IP network. In practice, this featurein the cell-level controller is in most cases implemented by program.There is no need for structural changes in the known cell-levelcontrollers in the preferred embodiment of the invention, but a desiredfeature can be included in the software of the controller.Correspondingly, the features of the preferred embodiment of theinvention can be implemented by program in the multicast controller.

Even though the invention has been explained in the above with referenceto examples in accordance with the accompanying drawings, it is apparentthat the invention is not restricted to them but can be modified in manyways within the scope of the inventive idea disclosed in the attachedclaims.

1. A method, comprising: transmitting multicast data packets in at leastone first multicast tree from one transmitter through a plurality ofmulticast controllers to a plurality of recipients, wherein onlyunidirectional downlink communication exists between the multicastcontrollers and the recipients; generating at least one second multicasttree reserved for control messages in an internet protocol networkbeginning from a network multicast controller and ending to one or moremulticast controllers at cell level; and the cell level multicastcontroller originates a message to join the second multicast tree, ifthe join message is received, then transmitting the control messagesfrom the network multicast controller along the at least one secondmulticast tree reserved for control messages to the at least onemulticast controller at cell level, the control messages comprisinginformation on the multicast transmission of a internet protocol networkand a command configured to connect to the at least one first multicasttree of the internet protocol network configured for multicasts.
 2. Amethod as claimed in claim 1, further comprising: connecting, whenconnecting to the internet protocol network, the one or more multicastcontrollers at cell level to the at least one multicast tree configuredfor the network control messages.
 3. A method as claimed in claim 1,further comprising: connecting, after receiving a control message fromthe network multicast controller through the at least one multicast treeconfigured for the control messages, the one or more multicastcontrollers at cell level to the at least one multicast tree configuredfor multicasts and defined in the control message.
 4. A method asclaimed in claim 1, further comprising: transmitting, after connectingto the at least one first multicast tree configured for multicasts, bythe at least one or more multicast controllers at cell level, packetsreceived through the one first multicast tree to at least one receiverin a cell.
 5. A method as claimed in claim 1, wherein the controlmessages further comprise information on an identifier of one or moremulticast groups.
 6. A method as claimed in claim 1, wherein the controlmessages further comprise information on a time of validity of thecontrol messages.
 7. A method as claimed in claim 1, wherein the controlmessages further comprise information on a sender authentication.
 8. Amethod as claimed in claim 1, wherein the control messages furthercomprise a receiver filter.
 9. A method as claimed in claim 1, furthercomprising: registering, after receiving a control message from thenetwork multicast controller, by the one or more multicast controllersat cell level, a recipient of a multicast defined in the controlmessage.
 10. A method as claimed in claim 1, further comprising:notifying, after receiving a control message from the network multicastcontroller, by the one or more multicast controllers at cell level,recipients of its cell that a multicast is available.
 11. A method asclaimed in claim 1, further comprising: notifying, after receiving acontrol message from the network multicast controller through the atleast one multicast tree configured for control messages, by the one ormore multicast controllers at cell level, recipients of its cell that amulticast must be received.
 12. A method as claimed in claim 1, furthercomprising: refraining, after receiving a control message from thenetwork multicast controller through the at least one multicast treeconfigured for control messages, from processing the control message bythe one or more multicast controllers at cell level.
 13. An arrangementfor implementing multicasting in internet protocol networks, thearrangement comprising: a plurality of routers configured to transmitdifferent components in the internet protocol networks to each other; atleast one first multicast tree configured to transmit multicast packetsthrough a plurality of multicast controllers to a plurality ofrecipients, wherein only unidirectional downlink communication existsbetween the multicast controllers and the recipients; a plurality ofcell-level multicast controllers configured to transmit packets to theplurality of receivers; and a network multicast controller that isarranged to control the cell-level multicast controllers, wherein aninternet protocol network comprises at least one second multicast treereserved for control messages and configured to route control messagesbeginning from the network multicast controller and ending to theplurality of cell-level multicast controllers, wherein the cell levelmulticast controller originates a message to join the second multicasttree, and if the join message is received, the network multicastcontroller is then configured to transmit the control messages along theat least one second multicast tree to the plurality of cell-levelmulticast controllers, and the control messages comprise information onthe multicast transmission of the internet protocol network and acommand configured to connect to the at least one first multicast treeof the internet protocol network configured for multicast transmissions.14. An arrangement as claimed in claim 13, wherein the cell-levelmulticast controllers are configured to connect to the multicast treeconfigured for network control messages when connecting to the internetprotocol network.
 15. An arrangement as claimed in claim 13, wherein thecell-level multicast controllers are configured to connect to themulticast tree of the internet protocol network configured formulticasts after receiving a control message from the network multicastcontroller through the multicast tree configured for control messages.